Bulk Acoustic Waves (BAWs) are generated in piezoelectric media by applying a time-varying electric source to a suitable electrode system. Often the source can be a sinusoidal or quasi-sinusoidal voltage. The electrode system varies according to the forms of the piezoelectric body, and the ensuing motion intended. Over the years, many shapes and modes of motion have been employed. Piezoelectric materials have been used either as a single body, or suitably attached to other substances to form composite shapes that yield the desired structure. The desired structure may comprise rods and bars, rectangular and circular plates, forks and so on. When used as an acoustic resonator or filter, one or more frequency-determining dimensions of the body are employed to yield the desired resonance characteristics. When used as a sensor, piezoelectric materials are exposed in aqueous solution measuring the loading effects from ambient molecules adhering to the device's surface.
One simple example of using a piezoelectric material as a sensor is a thin plate comprised of a single piezoelectric material with electrodes placed upon the major surfaces. When an electric voltage energizes the plate and the requisite piezoelectric coefficient exists, the plate will exhibit resonances when the exciting frequency is such that its thickness dimension is an integral multiple of an acoustic wavelength. In this situation, BAWs form standing wave patterns in the thickness direction; the standing wave pattern is comprised of two counter-propagating waves, each traveling in the thickness direction. As sensing ensues, the mass loading of the in situ solution constituents results in shifts in the resonant frequency spectrum that can be interpreted as “signatures” of the element's concentration in the solution.
Progression of sensing technology with time has been toward operation at increasingly higher frequencies and this tendency is apt to continue in the near future. Wavelength is inversely proportional to frequency, so one therefore finds increasing use of the thickness of piezoelectric materials as the frequency-determining dimension for BAWs. The structures take the forms of thin films and membranes in order to reach the high frequencies demanded by technological needs. Mechanical motions of this nature are referred to as thickness modes of vibration.
There are two primary methods of exciting BAWs in plates and other structures, and they differ in the direction of the exciting electric field. In one method, the exciting electric field is parallel to the direction of propagation of the acoustic waves, while in the other; the exciting electric field is perpendicular to the direction of propagation of the acoustic waves. When applied to the thickness modes of a plate, the acoustic plane waves propagate in the thickness direction; when electrodes are placed on the major surfaces of the plate, the exciting field is in the thickness direction. This excitation method is consequently called “thickness excitation” (TE) or “thickness-field excitation” (TFE). Electrodes placed at the plate edges to produce a field in a direction parallel to the plate surface, and hence perpendicular to the acoustic plane wave direction, produce what is referred to as “lateral excitation” (LE) or “lateral-field excitation” (LFE).
Another type of acoustic mode is the surface acoustic wave (SAW) and there are several varieties of these, the most often used is the Rayleigh surface wave. This type of wave has motion that is primarily confined within an acoustic wavelength or two of the surface of a planar structure, and propagates in a direction lateral to the depth of the substrate, i.e., in a direction along the planar surface.
When the SAW mode was first exploited in electronic technology, various methods were used to generate the SAW waves. One technique is to attach a BAW wave transducer to one slanted face of a prism bonded to a planar surface upon which it is desired to propagate the SAW. The BAW impinging on the surface at an angle converts a portion of its energy to a SAW, which then propagates along this surface, in the direction away from the prism.
Introduction of the interdigital transducer (IDT) resulted in an efficient method for generating SAWs. The IDT, moreover, is a planar electrode structure that can be applied by highly developed methods, such as photolithography, that are used in the microelectronics industry. The IDT has become the transduction means of choice for SAW applications, and has been developed in a variety of configurations depicted in FIGS. 1 and 2.
Referring now to FIGS. 1A–1C, the simplest IDT consists of two symmetrically interdigitated combs of identical electrode finger stripes, each comb consisting of geometrically parallel metallic fingers. FIG. 1A is a perspective view of an IDT piezoelectric plate 10, comprising a substrate 11, input transducer 12, input terminal pair 13 and 14, SAW, represented by wiggly line 15, output transducer 16 and output terminal pair 20 and 21. Arrow 17 indicates the preferred direction of propagation in the piezoelectric plate. The FIG. 1B cross-sectional view of substrate 11 depicts two sets of three finger pairs of IDT electrode fingers stripes 18 for illustrative purposes. The typical IDT generally includes hundreds of finger pairs. The FIG. 1C exploded view depicts one pair, or period 19, of IDT finger stripes 18 and a period gap, or separation, G, between the IDT fingers 18. The FIG. 1C width A between the centers of each IDT finger 18 is ½ λ, and the width of each IDT finger 18 is ¼λ, where λ is the acoustic wavelength, equal to the SAW velocity divided by the frequency of operation. Aluminum is often preferred as the electrode finger material because its acoustic impedance closely matches the acoustic impedances of most piezoelectric substrates. The IDT piezoelectric plate 10 depicted in FIGS. 1A–1C is a two-port structure, and the exciting voltage is placed across the two bus bars to energize the electrode fingers.
FIGS. 2A–2F depict a number of well-known IDT concepts and configurations. FIG. 2A illustrates a typical SAW configuration; the number of electrode fingers and transducer separation is related to bandwidth. FIG. 2B illustrates an alternative SAW configuration; where the number of fingers per period, or wavelength, is varied. FIGS. 2C and 2D illustrate a thinned transducer, or open structure, and dummy fingers, respectively. FIG. 2E depicts one and two port configurations. FIG. 2F depicts the concept of apodization in which changing the overlap of the fingers from opposite electrodes in a determined pattern results in weighting the frequency response of the transducer.
Another advantage of the IDT for SAW applications, is that the frequency characteristics, such as center frequency of a resonator and bandwidth of a filter are determined by the dimensions of the IDT, rather than the dimensions of the piezoelectric structure. This allows reaching high frequencies by photolithographic means on a robust substrate, rather than by reducing the thickness of a plate.
Despite the advantages of SAW operation briefly mentioned, for many applications BAW devices are preferable to SAW devices. Prior art BAW and SAW sensors that have relied upon the inclusion of a biologically active or chemically active layer, hereinafter bio-active or chem-active layer, respectively, specific to an antigen or chemical to be detected in an ambient environment. SAW sensors also present fabrication difficulties because two parallel, planar IDTs are required, with one IDT as a reference arm and the other IDT as a detection arm coated with a bio-active or chem-active layer. The prior art parallel IDT design is complex in both mask design and processing, because selective deposition of the active layer requires careful consideration so as not to corrupt the reference arm surface. Although BAW devices are generally simpler to design and fabricate, they have also demonstrated limitations as ambient mass loading, usually due to nonspecific binding agents, such as liquid or vapor phase water, that have reduced the sensitivity of these devices to a specific antigen or agent. Therefore, prior art BAW and SAW sensing arrangements are inadequate because of their reliance upon a specific a bio-active or chem-active layer for the substance of interest and the inherent complexities of the dual IDT parallel approach. Up until now, BAW quartz crystal microbalances (QCMs) using piezoelectric crystals as host assemblies for transducing the acoustic waves, (i.e., piezo-driving the acoustic waves) and substrate media for the bio-active or chem-active layers have been used for sensing bio-active and chem-agents, but the QCM is not always suitable because the bio-active or chem-agents usually do not selectively bind to the electrode material. To date, SAW devices have seen increasing use in the commercial world despite their sensitivity and processing limitations, and BAW sensors have languished due to traditional designs being unable to overcome the nonspecific mass-loading characteristic of such devices.
Additionally, there are a number of other difficulties to overcome in producing electrode structures for thickness shear, or extensional, excitation of BAW. One major difficulty is including a thin film active layer capable of bonding to select piezoelectric material major surfaces. Another limitation is the ability to augment the electrode to accommodate two or more channels for both detection and reference operation. Another problem in this area is the ability to operate at higher frequency regions (i.e., GHz). Up until now, BAWs generated from IDTs would be considered weak, spurious and detrimental because the typical low IC (integrated circuit) voltages do not provide adequate electric field strength for piezoelectric excitation of conventional electrode structures and result in unacceptable performance. Additionally, a non-uniform electric field further degrades performance. Traditional BAW sensors operate with phase progression perpendicular to the substrate surface and ambient solution mass loading tends to dampen the sensing response.
To overcome the disadvantages, shortcomings and limitations of prior art BAW sensors, a new BAW structure needs to exhibit a number of characteristics. The new BAW structure should produce BAWs that are essentially extensional plane waves, with propagation away from the substrate surface and phase progression substantially oblique to the substrate surface. This is a critical because prior art BAW sensors operate with phase progression substantially perpendicular to the substrate surface and mass loading (i.e., from an ambient solution) dampens the sensing response. The new BAW sensor structure should also produce a thickness electric field that is substantially uniform over a substantial portion of the structure's active area to virtually eliminate the sharp spikes depicted in FIGS. 3, 4, and 5, arising from the IDT structure, and mitigate the spurious readings from SAW sensors. The new BAW sensor structure should have a low voltage electric field that can be produced with the low voltages of around 10 volts or less typically found in integrated circuit (IC) chips.
Up until now, there has been a long-felt need for BAW sensors to overcome the disadvantages, shortcomings and limitations of reliance upon a specific active layer for the substance of interest, nonspecific mass loading and the inherent complexities of the dual IDT (parallel) approach. This invention fulfills this long-felt need with a BAW sensor structure coated with biological and/or chemical materials with multiple electrode depositions and a thin film piezo-transduction layer hosted on a substrate. By employing suitably placed coatings of biological or/and chemical active material, along with multiple electrode depositions that preserve the extensional wave propagation, on a thin film piezo-transduction layer hosted on a substrate the disadvantages, shortcomings and limitations of the prior art have largely been overcome and obviated. The BAW sensor structure of the present invention produces a low-voltage, substantially thickness-directed electric field and BAWs that are essentially extensional plane waves propagating away from the substrate surface having a phase progression substantially oblique to the substrate surface. The low-voltage, thickness-directed electric field produces essentially extensional BAWs that are also substantially uniform over a substantial portion of the BAW structure. Furthermore, the new BAW structure of the present invention is also compatible with IC processing techniques.